Harbour Island Force Main Replacement Project

The Harbour Island Force Main conveys more than 15 million gallons, or 30 percent of the City of Tampa’s wastewater each day from the Krause Street Pumping Station in Downtown Tampa to the treatment plant on Hookers Point.  Due to the deteriorating condition of the pipe and the critical service the Force Main provides to the City’s wastewater system, the City embarked on the Harbour Island Force Main Replacement Design-Build Project.

The City of Tampa selected the Design/Build team of Kimmins Contracting Corp and Wade Trim for the force main replacement.  Wade Trim as lead designers hired Brierley Associates to provide microtunnel specialist services including review of design plans and geotechnical studies, assist with interviews and bid evaluations for the microtunneling contractors, submittal reviews, and onsite supervision of the microtunnel construction activities.

The microtunnel component of the force main replacement consisted of the installation of a 54-inch diameter Hobas pipe transiting beneath the Ybor Channel Turning Basin from the Port of Tampa on the east to Cotanchobee Ft. Brooke Park in downtown Tampa on the west, a distance of approximately 3200 linear feet.  For the subaqueous crossing beneath the 38 ft deep shipping channel, the Microtunneling Contractor installed a 78-inch diameter steel casing pipe using 20 ft Tri-Lock pipe joints.  For access to the tunnel depth, the contractor installed a 40 ft diameter launch shaft and a 20 ft diameter receiving shaft each to depths of about 78 ft below ground surface (to El -72 ft).  Secant piles (42-inch diameter) were installed at both shafts, 55 for the launch shaft and 34 for the receiving shaft including 5 shafts on each side for the break-in / break-out walls.  A 6 ft tremie seal with micropile tension anchors installed “in the wet” allowed dewatering of the shafts.

The geology consists of about 20 ft of overburden material including fill is beneath the surface underlain by a mixed zone of clayey and sandy soils and weak limestone rock with isolated lenses of harder dolomite and chert inclusions.  The receiving shaft is excavated mostly in harder limestone/dolomite/chert encountered below El -25 ft.  The tunnel penetrated through mixed face conditions (clayey to sandy soils and weak rock) for the entire interval beneath the shipping channel.  The microtunneling contractor chose to use a face-accessible 78-in diameter MTS Perforator slurry microtunneling machine.  A hyperbaric chamber was installed behind the cutting face to allow diver interventions, which were not required.  A total of six (6) Intermediate Jacking Stations (IJS) were installed in the tunnel.